1. Field of the Invention
The invention relates generally to a method and apparatus for ultrafiltration during hemodialysis in which dialysis fluid is passed through a balancing device equipped with shut-off valves and balancing chambers connected to the dialyzer, and the ultrafiltrate is removed in an accurately predetermined amount.
2. Related Technology
West German Patent 28 38 414 (Oct. 31, 1984) and its counterpart U.S. Pat. Nos. 4,267,040 (May 12, 1981); 4,530,759 (July 23, 1985); and 4,770,769 (Sept. 13, 1988), the respective disclosures of which are hereby incorporated herein by reference, disclose a device which has a balancing device including two chambers separated by a movable element, each chamber having an inlet conduit for the introduction of fresh dialysis fluid and an outlet conduit for discharge of used dialysis fluid. Shut-off valves are disposed in the inlet and outlet conduits and are connected and switched (activated) by a control unit. A pump is disposed in the dialysis fluid flow circuit between the dialyzer and the balancing device to move the used dialysis fluid. A dialysis valve is provided in the dialyzer inlet conduit and an air separator is provided in the discharge conduit of the dialyzer.
This apparatus is operated in such a way that fresh dialysis fluid from a dialysis fluid source is introduced alternately to the two balancing chambers through appropriate switching of the shut-off valves in the inlet conduits. At the same time, fresh dialysis fluid is introduced from an already filled space in the other balancing chamber, as a result of which the toxins that are desired to be removed from the blood are removed by diffusion as the blood flows through the dialyzer. The dialysis fluid that becomes used dialysis fluid as a result of this diffusion is then pumped into a second space of the same balancing chamber from which the used dialysis fluid can be discharged.
A portion of the fluid circuit which is included between the balancing device and the dialyzer behaves as a closed system with a constant volume. In order to remove liquid from this system, a removal device is connected to a discharge.
The amount of fluid that is removed from the system with the aid of the removal device must be replaced by an equal amount of fluid based on the properties of the balancing device mentioned above, which amount of fluid goes from the blood side to the dialysis fluid side of the dialyzer membrane. Thus, the amount of fluid removed with the aid of the removal device corresponds to the amount of fluid that goes through the membrane of the dialyzer (i.e., the ultrafiltrate). The removal device, which behaves as an ultrafiltrate pump, is designed in such a way that control of the ultrafiltration is possible.
The balancing device and the removal device must be highly accurate and precise. In hemodialysis treatment, typically, about 200 liters of dialysis solution are passed through the dialyzer. The amount of ultrafiltrate is typically about 2 to 3 liters and must be able to be determined accurately with a deviation of the order of only 0.1 to 0.2 liter. Accordingly, the error which occurs in the balancing apparatus should not exceed an order of magnitude of 1 per thousand, if possible. In order to be able to remove an accurately predetermined amount of ultrafiltrate from the dialysis fluid circuit with the required accuracy, the removal device is equipped with a volumetric membrane pump, where each individual stroke of the pump corresponds to a unit amount of ultrafiltrate. The removal is done through a conduit from the lower part of an air separator disposed in the dialysis circuit, in order to ensure that only bubble-free fluid is removed. The outlet of the ultrafiltration pump or of the removal device is connected to the discharge conduit through a shut-off valve.
Although this prior art device operates accurately and reliably, it is desirable to simplify equipment of this type, especially to reduce the number of moving components and to minimize the need for calibration and safety-technological controls which must be carried out from time to time. These calibration processes and controls also concern the ultrafiltrate removal device.